1. Field of the Invention
The present invention relates to a digital loop carrier system located between a switch and a plurality of subscribers, and in particular to a remote digital terminal employed as part of a digital loop carrier system.
2. Related Arts
In general, a telephone company in North America provides services for subscribers who are scattered over a wide area. In this case, because on average the number of subscribers is low it is uneconomical to provide adequate services by establishing a separate telephone center having a switch for each of a small number of subscribers in a sparsely populated areas. Likewise, if telephone centers are installed for groups comprising economical subscriber numbers, and analog lines are used to connect the telephone centers and the subscribers, the lengths of the individual lines must be extended, a condition which is also not economical.
To resolve this problem, a digital loop carrier system (hereinafter referred to as a DLC system) has come to be employed. FIG. 12 is a specific diagram showing a DLC system. In FIG. 12, the DLC system comprises remote digital terminals (hereinafter referred to as RDTs) 1, which are installed at sites near those of subscribers; and integrated digital terminals (hereinafter referred to as IDTs) 2, which are installed at the telephone center. These RDTs and IDTs are connected by digital lines 3, and normally 28 digital lines are employed between each RDT and IDT.
An IDT 2 converts an analog signal, which is to be transmitted from switch, into a DS0 digital signal (64 bps), and transmits a DS1 digital signal (1.5 Mbps), which is obtained by multiplexing 24 DS0 signals, to the RDT 1 across the digital line 3. The RDT 1 separate the DS1 digital signal to analog signals for the individual subscribers, and distributes them to the subscribers via subscriber analog lines (subscriber channels). This series of procedures constitutes the downstream transmission process. For the upstream transmission, similarly, the RDT 1 converts analog signals received from the subscribers into DS0 digital signals, and then multiplexes the digital signals to produce a DS1 signal which it transmits across the digital line 3 to the IDT 2. Thereafter, from the received DS1 signal the IDT 2 separate DS1 signal to DS0 signals, and converts these signals into analog signals, and transmits them to the switch.
As DLC systems are installed between a switch and the subscribers for each group of subscribers in a specific area, provided is an communication network which efficiently connects the remote subscribers with the switch.
In a conventional DLC system, since they are provided on a one-to-one correspondence basis, an RDT 1 and an IDT 2 are installed as a pair. Therefore, the number of IDTs 2 installed at a telephone center corresponds to the number of RDTs 1 installed in a specific service areas, with the paired RDTs 1 and corresponding IDTs 2 being connected by digital lines 3. The reason that paired RDTs 2 and IDTs 1 are employed lies in the fact that RDTs 1 and IDTs 2 function independently, and were two IDTs 2 connected to a single RDT 1, the RDT 1 would not be able to identify the ID{circumflex over ( )}{circumflex over (T)} from which a DS1 signal is received.
To explain the following problem, which may be encountered during the operation of a DLC system, a specific diagram for explaining the objective of the present invention, FIG. 13, is used. Assume, for example, that the maximum number of subscribers that can be accommodated in an RDT 1 is 2000, and that when the number of subscribers serviced in a specific area is unexpectedly increased, the number of the subscribers handled by an RDT 1a and by an RDT 1b differ greatly, with 1900 subscribers, which is close to the maximum, being accommodated by the RDT 1a and 100 subscribers, which is much smaller than the maximum, being accommodated by the RDT 1b. 
In this case, the same subscriber variance also affects the IDTs 2a and 2b, which respectively correspond to the RDTs 1a and 1b. Although even when the number of accommodated subscribers is near the maximum, the IDTs 2 perform the minimum required performance, since the imposed load increases as the number of subscribers grows, performance is deteriorated. Furthermore, a situation wherein the number of subscribers accommodated by the IDT 2a is very large and the number of subscribers accommodated by the IDT 2b is very small is also not preferable because of operating stability and load balance requirements. Therefore, to maintain the performance of the IDTs 2 at a constant level or higher, load balances must be adjusted by distributing the subscribers equally between by the IDTs 2a and 2b. 
Specifically, the RDT 1a, which accommodates 1900 subscribers, is connected not only to the IDT 2a but also to the IDT 2b, where initially only 100 subscribers are accommodated (see broken line in FIG. 13), and, for example, of the 1900 subscribers accommodated by the RDT 1a 900 are now serviced by the IDT 2b. As a result, an equal number of 1000 subscribers each can be distributed to the IDTs 2a and 2b. 
However, as is described above, the RDT 1 and the IDT 1 are installed on a one-to-one correspondence basis, and a problem is that the RDT 1 can not be connected to a plurality of IDTs.
To resolve this problem, it is one objective of the present invention to provide a remote digital terminal (RDT) which can be connected to a plurality of integrated digital terminals (IDTs) included in a digital loop carrier (DLC).
To achieve the above objective, according to the present invention, Provided is a remote digital terminal (RDT) which connects digital lines extended from integrated digital terminals with subscriber lines extended from subscribers accommodated in the integrated digital terminals comprising:
first means for cross-connecting the digital lines and logic digital lines which are for identifying the digital lines on each of the integrated digital terminals;
second means for cross-connecting the subscriber lines and logic subscriber lines which are for identifying the subscriber lines on each of the integrated digital terminals; and
third means for cross-connecting the logic digital lines and the logic subscriber lines on each of the integrated digital terminals.
Specifically, the digital lines connected to each of the IDTs can be identified by the first means, and an arbitrary subscriber line can be allocated for an arbitrary logical subscriber by the second means. Furthermore, the first and the second means can be linked together by the third means, so that the IDT digital lines can be linked with the subscriber lines on the subscriber side, and the subscribers can be accommodated by an arbitrary IDT. Therefore, a plurality of IDTs can be connected to one RDT.
More specifically, to achieve the above objective, according to the present invention, provided is a remote digital terminal (RDT) which connects with digital lines extended from integrated digital terminals and subscriber lines extended from subscribers accommodated in the integrated digital terminals comprising:
a terminator for terminating a first digital line extended from a first integrated digital terminal and a second digital line extended from a second integrated digital terminal; and
a cross-connector for cross-connecting the first digital line and a first subscriber line extended from a first subscriber accommodated in the first integrated digital terminal, and for cross-connecting the second digital line and a second subscriber line extended from a second subscriber accommodated in the second integrated digital terminal.
In addition, the thus arranged remote digital terminal further comprises:
first data for corresponding first physical identifiers for physically identifying the first and second digital line with first logic identifiers for identifying the first and second digital line on each of integrated digital terminals;
second data for corresponding second physical identifiers for physically identifying the first and second subscriber line with second logic identifiers for identifying the first and second subscriber lines on each of integrated digital terminals;
third data for corresponding the first logic identifiers with the second logic identifiers; and
a processor for obtaining data for corresponding the first physical identifiers with the second physical identifiers based on the first, second and third data; and
wherein the cross-connector cross-connects the first and second digital lines with the first and second subscriber lines respectively based on the results obtained by the processor.
Other features and advantages of the present invention will become readily apparent from the following description when taken in conjunction with the accompanying drawings.